It has been proposed a prior art time-division multiplexing communication system, for example, as disclosed in JP2005-73075 A in which one or more slaves are connected in parallel with each other to a master by way of a two-wire signal line. The time-division multiplexing transmission system is provided for data transmission between the master and the slaves.
One example of the time-division multiplexing transmission system is proposed in a remote monitoring and controlling system as shown in FIG. 11. The remote monitoring and controlling system includes a transmission unit 21 as the master connected through the two-wire signal line Ls to an input terminal 22 and control terminals 23 collectively referred to as the slave. The remote monitoring and controlling system is provided to control loads L connected respectively to the control terminals 23 in accordance with monitoring inputs from a switch or sensor associated with the input terminal 22. The input terminal 22 and the control terminals 23 are given respectively with individual addresses.
The transmission unit 21 normally operates to cyclically vary an address data included in the transmission signal Vs to make constant polling to the input terminal 22 and the control terminals 23 sequentially. When receiving the monitoring input, the input terminal 22 transmits a monitoring data corresponding to the monitored input to the transmission unit 21. Upon reception of the monitoring data, the transmission unit 21 responds to transmit a control data corresponding to the monitored data to the control terminal 23 associated with the input terminal 22 by means of the address, in order to control the load L through the control terminal 23. A switch or sensor equivalent to the switch may be utilized to give the monitored input to the input terminal 22. Hereinafter, an explanation is made with a case in which the switch is utilized to give the monitored input to the input terminal 22. That is, the monitored input is generated in response to a manipulation of the switch, thus the monitored input is referred to as the manipulation input.
Incidentally, in a time-division multiplexing transmission system such as that described above, the slaves (the input terminal 22 and control terminals 23) communicate with each other via the master (the transmission unit 21) in accordance with a protocol of the polling/selecting system (to be referred to hereafter as a first protocol). Hence, a communication speed is comparatively low, and therefore this system is ill suited to the transmission of data having a much greater volume than monitoring data or control data such as an electric energy measurement value or a telephone call voice, for example.
Therefore, a communication system and communication method in which high-speed communication is performed while sharing a signal line Ls with a time-division multiplexing transmission system by modulating a voice signal and superimposing it onto a transmission signal Vs has been proposed (see JP H08-274742 A, for example). In the conventional example described in JP H08-274742 A, the voice signal is superimposed only during a period in which a series of pulses serving as a carrier is stable at a high level and a low level in order to avoid the effects of noise (harmonic noise) generated on a rising edge and a falling edge of the transmission signal Vs.
However, when the period in which the carrier (the series of pulses) of the time-division multiplexing transmission system is stable at a high level and a low level is set as a communication period for high-speed communication, as in the conventional example described in JP H08-274742 A, the communication period may be extremely short, depending on the condition of the transmission signal Vs (the transmission condition of the first protocol), and therefore this system may be unsuitable for transmitting a high-speed communication signal. When data transmission is performed during a period that is unsuitable for signal transmission, the noise that is generated on the rising edge and the falling edge of the carrier is more likely to have an effect, possibly leading to a reduction in the reliability of the data transmission.